It is well known to use a logic analyzer in the design of a microprocessor-based system. A conventional logic analyzer is used to display the system's response to various operations performed by the microprocessor, and to change the mode of operation of the microprocessor. The microprocessor interfaces with the logic analyzer through a personality module. Conventionally, the personality module has a probe cable which connects the module to the system under test (SUT), by way of the microprocessor's connection terminals. Recent microprocessors have as many as 68 closely spaced terminals, and it is extremely difficult to establish separate connections to the 68 different terminals.
Recently, microprocessors have been packaged on leadless chip carriers. A leadless chip carrier (described in JEDEC Standard MS-002) is a ceramic substrate with a pattern of planar contact pads flush with its underside. The substrate, with one or more integrated circuit (IC) chips mounted thereon and having their terminals connected to the contact the substrate, is fitted in a socket, or receptacle, which is provided with terminals that engage the contact pads and provide electrical connections between the ICs and other circuitry. The socket is designed to receive the chip carrier only, and it is not possible to introduce a probe into the socket without removing the chip carrier, or to replace the chip carrier in the socket if a probe is present therein.
A logic analyzer may be used to carry out in circuit testing of a microprocessor, i. e. monitoring of electrical events with the microprocessor connected as part of a microprocessor-based system and without the logic analyzer affecting operation of the SUT. In order to carry out in circuit testing of a microprocessor packaged on a leadless chip carrier in accordance with JEDEC standard MS002, a probe element must be fitted in the SUT's microprocessor socket and provision made elsewhere for accommodating the microprocessor itself. To facilitate making connections to all the terminals of the socket, the probe element would normally be of the same physical dimensions as the chip carrier.
U.S. Pat. No. 4,514,022 discloses a probe cable assembly which allows the microprocessor to be removed from the SUT and provides for both the personality module and the microprocessor to be connected to the SUT through the system's microprocessor socket, or receptacle. The probe cable has an instrument end, at which it is connected to the logic analyzer's personality module, and a probe end. A probe element is secured to the cable at the probe end, and is capable of fitting in the microprocessor socket of the SUT. The cable also comprises an auxiliary receptacle for receiving the microprocessor. The auxiliary receptacle has connection pins extending from its underside for connecting the microprocessor to other circuitry. The cable is formed from a flexible circuit board having three distinct portions, namely a main body portion and two leg portions. The auxiliary receptacle is carried on the main body portion with its connection pins projecting through the main body portion. A first of the leg portions has connection pads arranged in an array corresponding to that of the connection pins of the auxiliary receptacle, and is folded under the main body portion so as to bring its connection pads into contact with the connection pins of the auxiliary receptacle. The second leg portion carries a probe element that is configured to fit in a socket identical to the auxiliary receptacle, and is also folded under the main body portion so as to bring the probe element into position under the probe end of the main body portion. Signal conductor runs extend from the instrument end of the cable assembly to the connection pads on the first leg portion and to the auxiliary receptacle, and from the auxiliary receptacle to both the probe element and the probe end of the flexible circuit board. In one commercially-manufactured embodiment of the probe cable assembly, the flexible circuit board also has a ground plane comprising a single sheet of copper. Where the leg portions of the probe cable assembly are folded under the main body portion, there may be as many as four thicknesses of flexible circuit superimposed over one another. This results in the signal conductor runs being of non-uniform impedance along their length, and in electrical interaction among the different conductor runs. Moreover, the flexible circuit board is rendered quite stiff in places, making manipulation of the probe end inconvenient.
In addition, in the prior probe cable assembIy the contact pads of the probe element are connected to the runs of the cable by soldering, and these solder joints are subject to failure on repeatedly inserting the probe element into, and removing it from, the SUT's microprocessor socket.